Interpretive Summary: This paper describes laboratory and greenhouse experiments to find ways to reduce the amount of fungal spores needed to be applied at pathogens of weeds (bioherbicides) in order to provide effective weed management. First, the plant pathogenic fungus, Myrothecium verrucaria, was tested for the ability to survive in tank mix solutions of several commercially available herbicides. Several herbicides, particularly several common glyphosate-containing products were not compatible with the bioherbicide in tank mixes, so herbicides were evaluated for the growth inhibition of the fungus. Many herbicides failed to significantly slow the growth of M. verrucaria when tested at rates of up to 1% and all had 50% inhibitory concentrations of 0.3% or higher, indicating that residual amounts from sequential herbicide applications would not interfere with fungal growth. Finally, a series of surfactants were evaluated in vitro for their ability to improve the efficacy of M. verrucaria in laboratory and greenhouse assays against the weed, sicklepod. The best sicklepod weed control was achieved when M. verrucaria was combined with certain surfactants. Combinations of the bioherbicide with better surfactants and integration with synthetic herbicide programs may yield cost effective weed control.

Technical Abstract:
Herbicides and spray adjuvants were evaluated for compatibility with the bioherbicidal fungus, Myrothecium verrucaria. Several commercial formulations of glyphosate were found to be compatible for tank mixing with M. verrucaria, including Touchdown and RoundUp HiTech. Others, such as Accord XRT II and RoundUp WeatherMAX killed all the spores of M. verrucaria immediately after mixing at only 10% the maximum labeled application rate. Many herbicides, which were not suitable for co-application with M. verrucaria, did not inhibit the growth of the fungus when added directly to media at up to 1% concentration, indicating that these products would be available to use for sequential applications. Several commercially available spray adjuvants and polyoxyethylene tridecyl ether (TDA) formulations were tested in vitro for their efficiency in dispersing spores and in a plant bioassay for bioherbicidal activity. All of the products improved the activity of M. verrucaria over the water-only treatments and TDA formulations with a hydrophilic – lipophilic balance number of 8 or 10 has the highest activity. Better adjuvant selection and integration with affordable synthetic herbicides should aid in the development of more cost-effective biological control of weeds.